The invention relates to a method for evaluating a charged particle beam and a method for controlling a charged particle beam. Furthermore, this invention relates to an apparatus for the examination of specimen with a beam of charged particles.
Due to their high resolving power, beams of charged particles are used for the examination of specimen. Compared to optical light, the resolving power of a beam of charged particles is several magnitudes higher and allows for the examination of much finer details. Accordingly, charged particle beams, especially electron beams, are used for the inspection of masks and wafers used in semiconductor technology, which requires a very high resolution.
For an instrument using charged particles, for example a scanning electron microscope (SEM), it is important to characterize and to control the instrument""s quality of performance. Charged particle devices, like scanning electron microscopes, are sensitive to noise of all kinds, such as mechanical, acoustical or electromagnetic noise. Furthermore, internal instabilities and differences in the adjustments from one operator to the next may lead to a serious degradation of the system performance. Accordingly, the performance of the system has to be checked on a regular basis in order to guarantee an operation of the system according to the specifications. Especially, for those systems that are used to measure in line the quality of a production process, for example in the semiconductor industry, the performance of the entire system must be checked at least every day.
However, it is very difficult to determine the quality of system performance. Conventionally, the corresponding tests are performed manually by well-trained operators who can judge the system""s performance based on their own experience. The image quality, for example, is determined by comparing an image of a resolution target to a reference hard copy image. However, it is difficult to judge the quality of the resulting pictures by eye vision alone and each operator tends to judge picture quality by different criteria. Furthermore, the resolution of a system is normally determined by measuring the smallest distance between two edges in the image having the same background gray level between them. Searching for the smallest distance between two edges, however, does not lead to consistent results either, because the measurement locations are random and suitable measurement locations are very rare. Accordingly, the manual approach of evaluating the system""s performance is highly dependent on operator judgment and the chosen field of vision. It introduces operator inaccuracies and interoperator variations. Accordingly, there is a need for a more accurate, more consistent, and automated method for the characterization and/or the control of a charged particle beam.
The present invention provides a method for automatically evaluating the performance of a charged particle device. The method uses a signal coming from a reference resolution target. From this signal the spot size of the particle beam is determined. The method according to the present invention has the advantage that it does not depend on operator judgment. Therefore, the method leads to improved consistency, accuracy and reliability of the beam characterization, and, accordingly, to a better characterization of the quality of the system""s performance. Furthermore, the method according to the invention does not require an unique scan pattern on the sample, so that the method can be executed within existing instruments, for example scanning electron microscopes, with minimal modification. Suitable measurement locations are not dependent on a unique event (smallest distance between two edges). Accordingly, the results of the measurement do not depend on the chosen location. The spot size of the particle beam determines significantly the system resolution, image contrast and measurement quality. Accordingly, the determination the spot size of the particle beam leads to a good and very useful characterization of the charged particle beam and a very good quality control of the system performance.
Preferably, metal-on-carbon targets or porous silicon targets are used as a reference resolution target. Metal-on-carbon targets are basically contrast targets where small islands of metal with a high yield of secondary electrons are located on a surface of carbon with a low yield of secondary electrons. Metal-on-carbon targets basically do not have any additional surface topography which could have a negative influence on the determination of the spot size of the particle beam. Furthermore, metal-on-carbon targets have very sharp edges, which allows the determination of the beam spot size with high precision. Metal-on-carbon targets are also insensitive to charged particle bombardments, especially electron bombardments, and they exhibit a long term stability in their secondary electron emission behavior. These features make a metal-on-carbon target an excellent tool for the automatic determination of the beam spot size. Preferred metals for the use in a metal-on-carbon target include gold, tin or aluminum.
Porous silicon reference targets are basically topography targets where due to an aggressive etch treatment the silicon exhibits are highly structured surface including very steep edges. These very steep edges also allow the determination of the beam spot size with high precision. Porous silicon reference targets are also insensitive to charged particle bombardments, especially electron bombardments, and they exhibit a long term stability in their secondary electron emission behavior.
According to a further aspect of the present invention, there is provided a method for controlling a charged particle device. The method uses the above described evaluation of the system performance in order to find the correct parameter setting. Based on the improved consistency, accuracy and reliability of the beam characterization, the beam control also exhibits a higher level of consistency, accuracy and reliability, so that high quality measurements can be guaranteed over a long period of time without any interference from outside.
According to a still further aspect of the present invention, there is provided a target assembly for use in a charged particle apparatus, said target assembly comprises: a target support for supporting a target; a housing carrying the target support and having a higher thermal mass than the target support; and a heating element for heating the target. By heating the target, contaminates absorbed by the target are evaporated and thereby removed from the surface of the target. This guarantees a precise beam characterization over a long time period even in a production environment. The target is cleaned without the need to open the instrument. Accordingly, the down time of the instrument is reduced. Furthermore, the heating of the target reduces charging effects on the target. After the contaminates have been removed, the target cools down to room temperature, in order to perform the actual measurement.
By providing a housing, that carries the target support and that has a higher thermal mass than the target support, no excessive heat is transferred from the target support to neighboring elements and the surroundings. Accordingly, the heating of the target does not interfere with the normal operation of the instrument further reducing the idle time of the instrument